Abstract

We characterize a single beam supercontinuum “white light” trap and determine the trap stiffness in the transverse trapping plane. We realize a holographic white light trapping system using a spatial light modulator, and explore the generation of a dual beam trap and characterize its performance. We also demonstrate optical trapping and rotation of particles using a supercontinuum vortex beam. It is shown that orbital angular momentum can be transferred to spheres trapped in a supercontinuum vortex. Quantified rotation rates are demonstrated.

Figures (11)

Trap stiffness values for the direction parallel (kp) and perpendicular (ks) to the beam’s polarization as a function of wavelength for a 1µm polymer particle trapped in the focal plane of a Gaussian beam with a power of 11mW and a waist w0=1.75µm.

The visible component of the SC beam was selected using the infrared filter (IR filter) and expanded through an achromatic lens relay system, f1=100mm and f2=250mm. A dichroic beam splitter, DBS, reflected the visible light onto the back aperture of objective 1, 100x (Nikon NA 1.4) apochromatic microscope objective. A LED emitting at 950nm was used in a Köhler illumination scheme. This consisted of four f=50mm lenses, and two apertures, a, used to adjust the LED brightness and the field of view. A long working distance microscope objective (Mitutoyo, 100x, NA 0.7), objective 2, delivered the illumination to the sample plane. Light is reflected onto a CCD camera (Basler pl640-210gm) via an infrared mirror (M) and a 950nm interference filter, IF, (Comar 950IL25) to remove residual visible light. The spectrum of the supercontinuum source, shown as an insert, was recorded in the trapping plane immediately after objective 1.

(a). The set up used for holographic trapping was adapted from Fig. 2 as shown. The IR filter selected the visible wavelength component of the beam, and was expanded using achromatic lenses, f1=50mm and f2=250mm, to fill the SLM-spatial light modulator display area. Lenses f3 and f4 were used to image the reflected beam, and the first order was selected using an aperture positioned a focal distance away from f3 and f4. A 10° prism was used in the conjugate plane of the SLM, placed a focal length away from f4, to image the hologram produced from the SLM. (b) The dual white light spots (f3=630mm and f4=500mm), and (c) the supercontinuum LG beam (f3=200mm and f4=160mm), created are shown. Images were taken with a Watec-250D CCD.

Graphs (a) and (b) show the red, green and blue parts of the spectrum for the respective vertical and horizontal beam profiles of the upper trap, cross sections were taken using a camera plot, and the intensities have been normalized for ease of comparison. Graphs (c) and (d) show the vertical and horizontal beam profiles for the lower trap, again, the red, green and blue parts of the spectrum are shown. Figure 5(b) shows a picture of the dual traps.

Centered histograms of the x and y position of the two 2 µm polymer particles in the dual supercontinuum trap. Graphs (a) and (b) show the upper trap seen in Fig. 5, and graphs (c) and (d) show the lower trap. Red curves correspond to Gaussian fits using Eq. (9).

Rotation rate for three touching 1µm diameter spheres optically trapped in the first bright annular ring of the focal spot of supercontinuum LG beam (l=3, p=0). The LG beam has some higher order components, Fig. 5(c). The three trapped spheres and the tracking reconstruction are shown in the inset (see multimedia file 450kB).[Media 1]

(a). The LG beam waists as a function of l are shown with pictures of the supercontinuum LG beam at l=2, 4, and 5. (b) Particle rotation rates, Ω, for three touching 1µm polymer spheres are shown in black with increasing l. Theoretical points are shown in red with a line to guide the eye.